FIG. 1 shows the anatomy of a normal human ear. A normal ear transmits sounds through the outer ear 101 to the eardrum 102, which moves the bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea (or inner ear) 104 includes an upper channel known as the scala vestibuli 105 and a lower channel known as the scala tympani 106, which are connected by the cochlear duct 107. In response to received sounds the stapes, a bone of the middle ear 103, transmits vibrations via the fenestra ovalis (oval window), to the perilymph of the cochlea 104. As a result, the hair cells of the organ of Corti are excited to initiate chemi-electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Some patients may have partially or completely impaired hearing for reasons including: long term exposure to environmental noise, congenital defects, damage due to disease or illness, use of certain medications such as aminoglycosides, or physical trauma. Hearing impairment may be of the conductive, sensineural, or combination types.
Implants often include various electro-magnetic transducers that may function as an actuator, a sensor, and/or a switch. An example of an implant with an electro-magnetic actuator is a middle ear implant which mechanically drives the ossicular chain. Such a middle ear implant that includes a floating mass transducer was developed by Geoffrey Ball et al. (see U.S. Pat. Nos. 5,913,815; 5,897,486; 5,624,376; 5,554,096; 5,456,654; 5,800,336; 5,857,958; and 6,475,134, each of which is incorporated herein by reference).
Magnetic Resonance Imaging (MRI) examination may be contraindicated for a wearer of such an auditory (cochlear or middle ear) prosthesis since potential interactions between the implanted electro-magnetic transducer and the applied external MRI magnetic field may, at higher field strength (i.e. above about 1 Tesla), produce three potentially harmful effects:                1. The implanted magnet experiences a torque (T=m×B) that may twist the electro-magnetic transducer out of its position, thereby injuring the implant wearer and/or destroying the mechanical fixation.        2. Due to the external magnetic field, the implanted magnet becomes partly demagnetized and this may lead to damage or at least to a reduced power efficiency of the electro-magnetic transducer after exposure to the MRI field.        3. Radio frequency (RF) pulses (magnetic field B1 in MRI) emitted by the MRI unit can induce voltages in the coil(s) of the electro-magnetic transducer and this may destroy the transducer and/or may harm the patient.        
Because of these risks it may be generally forbidden to undergo (at least high-field) MRI examination for patients with an implant with electro-magnetic transducer. This may exclude the patient from certain important diagnosis methods.